Linear Generator Systems for Wave Energy Converters

[Pages:27]Linear Generator Systems for Wave Energy Converters

Jochen Bard, Peter Kracht, Fraunhofer IWES

Deliverable D3.2

Report on Linear Generator Systems for Wave Energy Converters

by Jochen Bard, Peter Kracht,

Fraunhofer IWES

September 2013

? Fraunhofer IWES

1. Introduction

In the course of the research project "Structural Design of Wave Energy Devices" (SDWED) a study on the different technologies used as linear generators in wave energy converters was conducted. This report gives the main results of this study.

Since the 1970ies various different designs and working principles for wave energy converters have been proposed, out of which a few were actually build, deployed and sometimes successfully tested. Nonetheless wave energy conversion is still an immature technology. In order to develop it further towards commercialization mainly two aspects need to be addressed. Firstly survivability in the extreme environment must be achieved at reasonable costs and secondly the overall efficiency of the devices needs to be increased. Accordingly research addresses fields like the structural design of the converters, the working principle, moorings etc.

Also the generator system of the converters is a topic of on-going research. Some wave energy converters, e.g. oscillating water column or overtopping WECs, use rotating prime movers, allowing the application of rotating electrical machines for conversion from mechanical to electrical energy. Rotating electrical generators are widely used in conventional energy production and also in renewable energy systems like wind energy converters. A different situation is found, if one looks at generator systems for a WEC following for instance the working principle of a point absorber, converting the incident wave force into a translational movement of body. In this case some kind of linear actor is required for the next conversion step. Currently mainly hydraulics and electrical linear generators are discussed as candidate system for the application in this type of wave energy converters. But also alternative technologies like artificial muscles and mechanical conversion systems are proposed. Some of these technologies are used in various other fields, but only limited knowledge and experience on the application in WEC exists. Currently it's still unsure, which of the various generator technologies will finally make the race. Therefore it was found necessary to compare as many different options as possible in order to give device developers an overview of the options and a chance of a basic comparison of the different technologies.

In the following the results of the study are given. Chapter two introduces the point absorber wave energy converter, which is one of the most studied concepts. Besides the wave energy converter itself some options of how to control the converter regarding energy optimisation are discussed and it's explained how these control strategies effect the requirements on the power take-off (PTO) and the generator system respectively. In the following chapters different options for the generator system are discussed. Corresponding to the actual state of research in the area, some are discussed in some more detail, these are namely the hydraulic system and electrical generators and additionally the artificial muscle. Some other technical options for the PTO systems are rather briefly introduced, which is due to the fact that they are not so widely discussed and thus less literature could be found on this technologies. Nonetheless also these systems might be candidate systems, whereas further research is required to understand their characteristics in terms of application in WECs. Finally a summary and brief comparison on the different options is given.

2. Characteristics of Wave Energy Devices of the Point Absorber Type

2.1 Working principle

In the following different options for linear generators in WECs are introduced, discussed and compared. Obviously the choice of the generator system highly depends on the WEC characteristics such as sizing, design sea state climate and working principle. In Figure 1 a schematic view of a simple point absorber type WEC is depicted. The converter consists of a floater, connected to the sea-bed by some sort of mechanism ? termed power take-off system in the following. The extraction of wave energy is described by the following principle: "The physical law of conservation of energy requires that the energy-extracting device must interact with the waves such as to reduce the amount of wave energy that is otherwise present in the sea. The device must generate a wave, which interferes destructively with the sea waves" [1]. The role of the power takeoff system in the overall system is to provide two functionalities. Firstly the power take-off system gives the means to control the device and thus to optimize the energy transferred from the incident waves to the WEC. Secondly the power take-off system converts the mechanical power from the floater to electrical power delivered to the grid. The power take-off characteristics required to fulfil both functionalities are derived in the following.

Figure 1: Heaving body reacting against fixed anchor. The indicated pump represents a hydraulic power take-off machinery (Source [2])

2.2 Control

To maximize the power extracted from the waves, PEX, the control of the device is key. In principle the power transferred is maximized, when the device is controlled to be in resonance with the incident waves. Often linear models are used to investigate control strategies, aiming at optimizing PEX [3], [4]. The simplest model used is a second order model, which is also used here as it is sufficient to explain the impact of control strategies on the PTO system. As an example in Figure 2 the electrical analogue of a second order model for a point absorber wave energy converter is depicted. The mechanical analogue would comprise the wave excitation force, mass, spring and damper.

Figure 2: Electric analogue of a point absorber wave energy converter ([3])

The power captured is maximized by controlling the PTO in a way that the eigenfrequency of the system meets the frequency of the wave excitation force and the damping applied by the PTO equals the natural damping of the WEC (R = RL in Figure 2). This approach to control a WEC is termed reactive or conjugate complex control. In [5] the theory is discussed in detail, covering also more complex models of the WEC and irregular waves. As mentioned by applying reactive control PEX is maximized. Nevertheless this control approach is sometimes assumed inadequate [3]. This is due to the impact on the PTO system. A reactive control, optimal in terms of maximizing PEX, results in:

- Reversed power flow in the PTO system during parts of the wave cycles, - Large displacements of the floater and thus the PTO system, - Extremely high peak forces in the PTO system and - A high ratio of peak to mean power.

As will be described in the latter all four aspects show a negative impact on the performance of the PTO system. Therefore another control approach, termed linear damping is often proposed. The idea behind this approach is to remove XL in Figure 2, leading to a passive behaviour of the PTO system. Positive characteristics of this approach regarding the PTO system are:

- No reversed power flow required, - Reduce displacements of the floater, - Reduced peak forces (but still very high) and - Lower (but still high) peak to mean ratio.

On the other hand PEX is only optimal if the natural frequency of the device equals the frequency of the incident waves, which only occurs for one sea-state. This means by applying linear damping PEX will definitely be lower than in the case of optimal reactive control, which needs to be weighed against the advantages of applying linear damping. Obviously a combination of both control approaches is possible. Applying only limited reactive power holds the possibility to optimize the overall system in terms of efficiency, rating of the PTO etc.

Note: An alternative control strategy originally proposed by Budal et al [6] would be the so-called latching. For reasons of simplicity latching and further alternative control strategy are not considered in this report.

3. Requirements on the generator system

As in almost all applications the requirements on a drive train system derive from various aspects of the application. In the given case it appears to be reasonable to group the requirements according to their origins. In the following two groups are introduced, which are working principle dependent and general requirements, and illustrated by examples.

3.1 Working principle dependent requirements

The requirements on the generator system deriving from the working principle are manifold. In the following it's described how the working principle affects the choice of the generator system.

Life cycle times To extract power the floater follows the water surface elevation for each wave cycle. This obviously leads to an extreme number of cycles over the life time of a device. In [7] the development of a hydraulic cylinder for the Oyster WEC [8] is described. The cylinders are tested against an assumed 25 million cycles during a service period of five years.

Force/Speed characteristics Though the displacement of the floater and the force from the PTO system highly depends on the control approach, it can be said, that in general a PTO system designed for high forces and low displacement/speed is required. In [9] peak velocities in the range of 0.5 to 2 m/s are assumed for a direct generator in a WEC of the point absorber type. If for instance a device designed for a peak power of 100 kW is considered a peak force of 100 kN would be required at a velocity of 1 m/s.

Peak to average power ratio Also the peak to average power ratio highly depends on the control approach. Figure 3 and 4 show a comparison of instantaneous and average power of a point absorber WEC in the same sea state applying linear damping and a control, including a reactive component, respectively [3].

Figure 3: Instantaneous and average power using linear damping (source [3]). Figure 4: Instantaneous and average power using a control, including a reactive component (source [3]).

It can be seen how by applying linear damping the peak to average power ratio can be significantly decreased (see Figure 3). But obviously in any case the average power is significantly less than the instantaneous power (see Figure 3 and 4). In [3] peak to average ratios of 7.7 ? 17.1 in the case of linear damping and 25.2 ? 58.3 in case of the control with reactive component respectively have been found. From the high peak to average ratios two requirements on the PTO system derive. Firstly the PTO system should have a high overload capacity. This would allow for operating the PTO system in overload conditions during the relatively short extreme power peaks, which would result in reduced investment costs. Secondly a high part-load efficiency is required. This is due to the fact, that the PTO will be operated in part-load during large time spans. Typically components ? no matter if hydraulics, electromechanical or alternative systems are considered ? show the highest efficiency at or closed to the design point (see Figure 5). This results in a poor efficiency over large time spans.

Figure 5: Typical efficiency vs. load factor curve of an electromechanical PTO system (source [3]).

Power flow in both directions From Figure 3 and 4 it can be seen that, if reactive control or a mix of linear damping and reactive control is applied, a reversed power flow in the PTO system is required. This must obviously be accounted in the choice of the generator system. Additionally it must be considered, that in the case of reactive control a high efficiency and full and part-lad is paramount, as the reversion of the power flow, results in additional losses in the PTO.

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